Unlike the ethyl and higher trialkylaluminum compounds which can be made economically by reactions of aluminum, hydrogen, and an olefin, trimethylaluminum has been produced only by processes which begin with a methyl halide. These include the direct reaction of a methyl halide with aluminum metal to form the methylaluminum sesquihalide, followed by a reduction step generally utilizing sodium as the reducing agent. Thus, starting with methyl chloride as a source of the methyl group: EQU 3CH.sub.3 Cl+2Al.fwdarw.(CH.sub.3).sub.3 Al.sub.2 Cl.sub.3 EQU (CH.sub.3).sub.3 Al.sub.2 Cl.sub.3 +3Na.fwdarw.(CH.sub.3).sub.3 Al+3NaCl+Al
This process has been used on a commercial scale to produce trimethylaluminum. Processes of this type are described in an article by A. V. Grosse and J. M. Mavity, Journal of Organic Chemistry, 5, 106 (1940), and in U.S. Pat. Nos. 2,863,894 and 2,954,389.
U.S. Pat. No. 2,744,127 describes a related method involving the direct reaction of a 40Al/60Mg alloy with a methyl halide, according to the equation: EQU CH.sub.3 Cl+Al.sub.2 Mg.sub.3 .fwdarw.2(CH.sub.3).sub.3 Al+3MgCl.sub.2
A method described in U.S. Pat. No. 2,839,556 does not use a reducing metal but is based on formation of cryolite as a means of removing halogen from a methylaluminum halide. For example, EQU (CH.sub.3).sub.2 AlCl+NaF.fwdarw.(CH.sub.3).sub.2 AlF+NaCl EQU 3(CH.sub.3).sub.2 AlF+3NaF.fwdarw.2(CH.sub.3).sub.3 Al+Na.sub.3 Al.sub.2 F.sub.6
All of the above-described methods have the disadvantage of forming very large amounts of inorganic metal halide by-products. These materials not only have very low value, but are also generally produced in forms which makes their recovery uneconomical. Hence they must be disposed of in a safe and ecologically acceptable manner, which adds further economic penalty to the trimethylaluminum synthesis.
Reviews of organoaluminum compound synthesis, e.g., in "Organoaluminum Compounds" by T. Mole and E. A. Jeffery (Elsevier, N.Y., 1972) describe other methods of trimethylaluminum synthesis generally not useful for economic commercial production. These include the initial preparation of a Grignard reagent, CH.sub.3 MgX, and its reaction with an aluminum halide in an ether solvent, EQU 3CH.sub.3 MgX+AlX.sub.3 .fwdarw.(CH.sub.3).sub.3 Al+3MgX.sub.2
which cannot be removed readily from the trimethylaluminum product. Another route, which has been of academic interest only, is initial synthesis of very toxic dimethylmercury (from a CH.sub.3 MgX reagent), from which the mercury can be displaced by aluminum in a solvent-free reaction. EQU 3(CH.sub.3).sub.2 Hg+2Al.fwdarw.2(CH.sub.3).sub.3 Al+3Hg
A recent patent, U.S. Pat No. 4,118,409, provides for jointly making trimethylaluminum and alkylaluminum bromides and iodides in an alkyl exchange process by mixing an aluminum trialkyl, such as triethylaluminum, and a methylaluminum bromide or iodide and then distilling from the mixture trimethylaluminum as a first fraction and then alkylaluminum bromides or iodides as a subsequent fraction.
Still more recently S. P. Diefenbach has described several methods for making trimethylaluminum from triethylaluminum. In U.S. Pat No. 4,364,872, triethylaluminum is reacted with a methyl halide in the presence of a catalyst formed from a bismuth compound, e.g. BiCl.sub.3. The reaction is conducted in an autoclave.
Diefenbach U.S. Pat No. 4,364,873 describes a similar process using a catalyst formed from a vanadium compound (e.g. VOCl.sub.3), a trialkylaluminum (e.g. triethylaluminum) and an alkyl iodide.
Diefenbach U.S. Pat. No. 4,364,474 describes a trimethylaluminum process using a non-catalyzed alkyl exchange between a higher trialkylaluminum such as triethylaluminum and methyl iodide.
Of the foregoing alkyl exchange systems, the most cost-effective process appears to be that shown in U.S. Pat. No. 4,364,872. Although the process of U.S. Pat. No. '872 is very effective on a laboratory scale, certain problems are encountered in conducting the process on a commercial scale. In U.S. Pat. No. '872, all of the tri-C.sub.2+ alkylaluminum and methyl halide is present together with the bismuth catalyst at the start of the reaction in a sealed autoclave. As the reaction proceeds, methyl halide is consumed and C.sub.2+ alkyl halide forms so the ratio of alkyl halide to trialkylaluminum is always high during the process. Alkyl halide can react with trialkylaluminum to form various alkylaluminum halides which consumes either trimethylaluminum product or the higher trialkylaluminum used as a reactant forming alkylaluminum halide.
Of greater concern in reaction mixtures containing high ratios of alkyl halide to alkylaluminum compound, is the possibility of a thermal excursion in a large scale reactor which can lead to an explosive reaction between the alkylaluminum compounds and the alkyl halides.